From OpenNMS

The notion of category is extensive in OpenNMS and is subdivided into several contexts.

SLM / RTC Categories

These categories are used for grouping interfaces and services for purposes of Service Level Management (SLM). Availability is maintained separately per category and is represented as a percentage of time during which the category's member services were available during a configurable sliding window in time. By default this window is 24 hours in length. The SLM availability numbers are most commonly displayed in the real-time console (RTC) on the front page of the web UI, but are also used in the creation of historical availability reports. SLM categories are maintained by the RTC daemon or RTCd.

SLM categories are declared in the file etc/categories.xml and referenced in the file etc/viewsdisplay.xml.

Surveillance Categories

These categories operate much like tags in the Web 2.0 sense. Many nodes can be members of one surveillance category, and any node can be a member of many surveillance categories. Surveillance categories are used as the basis of Surveillance Views, which are in turn used to scope a given user's view of the OpenNMS Dashboard. These categories can also be used in Filters and Rules, including the rules used to define SLM categories.

Data about surveillance categories themselves as well as about node membership in surveillance categories is maintained in the SQL database, in the categories and category_node tables.

Asset Categories

This kind of category is defined as part of a node's asset data. There are four asset categories as of this writing:

1. Display category (keyword displayCategory)
2. Notification category (keyword notifyCategory)
3. Polling category (keyword pollerCategory)
4. Threshold category (keyword thresholdCategory)

Like surveillance categories, asset categories may be used in Filters and Rules. Their use is not restricted to the set of uses implied by their respective names.